In the Local-Area-Networking (LAN) industry, Unshielded-Twisted-Pair (UTP) cables are predominant. UTP cables are used because the twisting provides high immunity to electromagnetic interference (EMI) and electromagnetic compatibility (EMC). Further, because they are unshielded, UTP cables provide isolation between stations that might have unequal ground potentials and thus prevent ground loops between such stations.
For 10-Gigabit-Ethernet-on-UTP solutions being considered by the IEEE, there is a problem associated with UTP that may limit feasibility and increase cost of the silicon being considered. This problem is typically referred to as Alien-Near-End Crosstalk (ANEXT) which is noise/interference that comes from electrical signals on adjacent UTP cables and typically cannot be eliminated by a transceiver using correlated noise cancellation circuits.
In addition, Near-End Crosstalk (NEXT) caused by adjacent transmitters also causes interference, but is typically less of a problem as associated electrical signals are correlated to a reference source that a transceiver can use to cancel much of the interference.
There are types of shielded cables that would substantially reduce the effects of this interference, but because the shielding is connected to ground at both ends of a cable, these cables may generate ground loops.
A ground loop exists when two pieces of equipment, which are on different power circuits and are referenced to different ground potentials, are connected together with a cable having a shield that connects the equipment shields' grounds together with low DC impedance.
Because of this problem with ground loops, the US LAN industry has traditionally supported UTP. In Europe, where Shielded-Twisted-Pair (STP) cables are more common, extensive management of power grids (to maintain equal ground potential from one location to another) is typically required to suppress ground looping. Europe has also adopted a 100-ohm UTP look-alike cable that contains a light foil shield (FTP) and that utilizes a common RJ-45 connector and is field terminable. However, because it is shielded, the UTP look-alike cable typically has the same problems as STP cables with respect to ground loops.
Referring to FIGS. 1 and 2, a conventional shielded modular plug 10 for terminating a shielded multi-pair communication cable 14 is illustrated. Cable 14 comprises an insulating sheath 16 enclosing a conductive cable shield 15 that, in turn, encloses four pairs of conductors or wires 18, each wire pair or signal pair twisted together (not shown) and forming a respective signal path during use. The construction of plug 10 is well known and generally comprises a dielectric housing 19 having a closed forward free end 22, a cable-receiving rearward end 24, a terminal receiving side 26 and a cable-receiving cavity (not shown) extending longitudinally from the rearward end 24 of the housing 19 to the terminal receiving side 26.
The plug 10 further includes a conductive shield portion 20 that electrically contacts the cable shield 15 when, as seen in FIG. 2, the plug 10 receives the cable 14. Eight parallel slots 28 defined by corresponding fins 29 open on to the terminal-receiving side 26 of housing 19 for receiving flat contact terminals 30. The eight slots 28 are aligned over a planar array of respective longitudinally extending wire-receiving parallel passages (not shown) which communicate with the cable-receiving cavity and which receive the ends of respective cable wires 18. Each flat contact terminal 30 is inserted into and fixed within an associated terminal-receiving slot 28 to terminate a respective wire 18 located in a respective wire-receiving passage.